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Two swarms of microscopic cometary dust blasted NASA's Stardust spacecraft in short but intense bursts as it approached within 150 miles of Comet Wild 2 last January, data from a University of Chicago instrument flying aboard the spacecraft has revealed.

"These things were like a thunderbolt," said Anthony
Tuzzolino, a Senior Scientist at the University of Chicago's Enrico
Fermi Institute. "I didn't anticipate running into this kind of
show." Tuzzolino and Thanasis Economou, also a Senior Scientist at
the Fermi Institute, will report their findings in the June 17 issue
of the journal Science.

The materials streaming from a comet range in size from
particles that could fit on the head of a pin to
boulders the size of a truck. Stardust mission planners correctly
estimated that their spacecraft could safely avoid the hazardous
larger objects by passing the comet at a distance of approximately
150 miles and using very effective dust particle shields.

Based on the data collected by the Dust Flux Monitor
Instrument, Tuzzolino and Economou estimate that NASA achieved its
goal of collecting at least 1,000 samples measuring at least
one-third the width of a human hair or larger during the flyby.

The Stardust spacecraft is scheduled to return the samples to
Earth in January 2006. Scientists will study the samples, the first
ever returned to Earth from a comet, for insights into the early
history of the solar system.

The Dust Flux Monitor Instrument collected data for 30
minutes when the spacecraft passed closest to the comet last Jan. 2.
Stardust encountered the first swarm of dust particles when the
spacecraft passed within 146.5 miles of the comet's nucleus. The
monitor detected a second intense swarm after passing the comet when
the spacecraft was approximately 2,350 miles from the nucleus.

"We believe that we see fragmentation of large dust lumps
into swarms of small particles after they are coming out from the
nucleus," Economou said.

In between the particle swarms, the impact of which lasted
just a few seconds each, the dust monitor went for periods of several
minutes before it detected another particle.

This isn't Tuzzolino's first encounter with a comet, though
it is by far the closest. He helped design, build and test the Dust
Counter and Mass Analyzer instrument that passed Comet Halley at a
distance of 5,000 miles or more in 1986 aboard two Soviet Vega
spacecraft. Halley had emitted a spray of dust "much smoother" than
that of Wild 2, Tuzzolino recalled.

"In general, one thinks of a comet as emitting gas and dust
in a nice, uniform steady state, sort of like a hose," he said.
Halley did show fluctuations, "but not to this extent."

The dust monitor detected its first impact when Stardust was
1,010 miles from the cometary nucleus. The last impact was recorded
at a distance of 3,500 miles as the spacecraft sped away. During one
intense event, the dust monitor detected more than 1,100 impacts in
one second. The largest particle measured during the cometary flyby
measured an estimated 500ths of an inch in diameter.

A similar instrument to the University of Chicago Dust Flux
Monitor Instrument is a component on NASA's Cassini mission to
Saturn. Cassini's High-Rate Detector, which Tuzzolino also built, is
part of a larger instrument, Germany's Cosmic Dust Analyzer, which
will study the ice and dust particles that form the major components
of Saturn's ring system. Cassini is scheduled to become the first
spacecraft ever to orbit Saturn on June 30.